It is becoming apparent that electronic systems, which combine new storage technologies with electrical drive engineering, will increasingly be used in the future in both stationary applications, as, e.g. wind turbines, and motor vehicles, such as hybrid and electric vehicles. In conventional applications, an electric machine, which, for example, is embodied as an induction machine, is controlled via an electric energy converter in the form of an inverter. A distinguishing characteristic for such systems is a so-called DC intermediate circuit, via which an energy storage unit, typically a battery, is connected to the DC side of the inverter. In order to be able to fulfill the power and energy requirements given for a respective application, a plurality of battery cells are connected in series. Because the current provided by an energy storage unit of this kind has to flow through all of the battery cells and a battery cell can only conduct a limited amount of current, battery cells are often additionally connected in parallel in order to increase the maximum current.
Besides a high total voltage, the series circuit consisting of a plurality of battery cells creates the problem that the entire energy storage unit fails if a single battery cell fails. This results from battery current no longer being able to flow. Such a power outage of the energy storage unit can lead to a power outage in the entire system. In a motor vehicle, a power outage of the drive battery can lead to the “shutdown” of the vehicle. In other applications, as e.g. the rotor blade adjustment of wind turbines, damage to the wind turbines can even occur when unfavorable framework conditions exist, such as, for example, strong wind. For that reason, a high level of reliability of the energy storage unit should always be the objective of the technology, wherein “reliability” describes the capability of a system to work error-free for a predefined period of time.
In the case of the simple series circuit of a plurality of battery cells, the large spread of the voltage range across the varying states of charge of the battery cells furthermore leads to restrictions when designing the remaining system components in terms of efficiency, installation space and costs. Hence, the electric machine must, e.g., be designed such that a required output can also be supplied at the lower voltage limit, i.e. when the battery charge is low. On the other hand, said electric machine must be able to withstand the operation at the upper voltage limit, i.e. when the battery is fully charged.
The American patent application US 2002/0175644 A1 discloses a system for controlling a three-phase electrical machine, which has a controllable energy storage unit comprising DC voltage sources that can be connected and disconnected as well as an inverter connected downstream.